The present invention relates to a method for forming a dot element, a semiconductor device using the dot element and a method for fabricating the device. More particularly, the present invention relates to a method for forming a dot element out of an ultrafine particle of the size of several nanometers and functioning as a quantum dot element, a semiconductor device using the dot element and a method for fabricating the device.
Currently, a ULSI is formed by integrating a great number of MOS devices on a single chip. In general, as an MOS device is miniaturized, the performance thereof is enhanced correspondingly. However, if the gate length thereof is 0.1 xcexcm or less, then the device can hardly operate normally as a transistor, because such a size is a physical limit for the device. A single-electron tunneling device, called a xe2x80x9ccoulomb blockadexe2x80x9d, has attracted much attention, recently as a candidate for breaking through such a limit (Kenji Taniguchi et al., FED Journal, Vol. 6, No. 2, 1995). In principle, a single-electron tunneling device performs logical operations and storing operations by controlling the movement of individual electrons, and is very effective in reducing power consumption. However, in order to form a single-electron tunneling device, semiconductor or metal fine particles of the size of several nanometers, called xe2x80x9cquantum dot elementsxe2x80x9d, are required. As disclosed in Japanese Laid-Open Publication No. 9-69630, for example, if a large number of Au dot elements are formed out of Au fine particles by sputtering or the like between metal electrodes formed on a substrate, then the Au dot elements form multiple bonds with each other, thereby realizing single-electron effects. In accordance with this method, however, it is very difficult to accurately control the positions where the Au dot elements are formed.
Thus, Sato et al. proposed another method for forming a dot element. In accordance with the method of Sato et al., 3-(2-aminoethylamino)propyltrimethoxy silane (APTS) is deposited on a substrate on which a PMMA resist pattern has been formed. Then, APTS on the PMMA resist is partially lifted off together with an unnecessary portion of the PMMA resist, thereby selectively leaving APTS at desired positions on the substrate. Thereafter, Au fine particles are attached onto only APTS, thereby forming Au dot elements.
Aside from the single-electron tunneling device, a different method for breaking through the limit of a device size using dot elements was also proposed. For example, S. Tiwari et al. disclosed in IEDEM Tech. Digest, 521 (1995) that an operating voltage would be lowered by using dot elements of silicon fine particles for the floating gate of a nonvolatile memory or the like. Tiwari et al. suggested that silicon dot elements could be formed directly on a substrate by performing a CVD process on accurately controlled conditions.
However, the methods of T. Sato et al. and Tiwari et al. have the following problems.
To control the positions of dot elements on a substrate by the method of T. Sato et al., the process steps of forming a PMMA resist pattern or the like on the substrate and then lifting off APTS with unnecessary portions of the PMMA resist pattern are required. Thus, the fabrication process is adversely complicated. In addition, in this method, the Au dot elements are formed onto APTS by utilizing the polarization of charges. Accordingly, if charges have been polarized at other sites on the semiconductor substrate, then Au fine particles are unintentionally attached to such sites. Therefore, it is not always possible to selectively form the Au dot elements only at desired sites.
On the other hand, in accordance with the method of Tiwari et al., silicon dot elements are directly formed on a substrate by a CVD technique. Thus, it is very difficult to control the sizes and positions of such dot elements on the substrate.
Because of these inconveniences, it is now hard to use dot elements, formed by the conventional methods, as a member of a semiconductor device or as quantum dot elements, in particular. That is to say, in accordance with the conventional methods, a semiconductor device, including dot elements formed with the sizes and positions thereof accurately controlled, is very much less likely to be realized.
An object of the present invention is to provide a method for forming dot elements while accurately controlling the sizes and positions thereof by taking various measures to precisely control the positions and sizes of fine particles over a substrate. Another object of the present invention is to provide semiconductor device of various types, each including the dot elements functioning as quantum dot elements as a component.
A first method for forming a dot element according to the present invention includes the steps of: a) forming a first compound on a part of a substrate; b) attaching a second compound to the surface of a fine particle, the second compound having such a nature as to be combined with the first compound formed on the substrate; c) combining the first and second compounds together and selectively placing the fine particle only on the part of the substrate where the first compound has been formed, thereby forming a dot element out of the fine particle.
In accordance with the first method, the positional accuracy of the dot elements can be controlled based on the position of the first compound formed on the substrate. In addition, only by selecting fine particles of a desired uniform size from the beginning, the sizes of the dot elements can be easily controlled. Accordingly, the positions and sizes of dot elements can be accurately controlled by performing a simple process, without any need for complicated process steps. As a result, dot elements, functioning as quantum dot elements in a device, can be practically formed.
In one embodiment of the present invention, both the first and second compounds are preferably organic compounds.
In another embodiment of the present invention, one of the first and second compounds may be an antigen and the other may be an antibody of the antigen.
In such an embodiment, a dot element can be formed such that the fine particle is fixed at a desired position, not undesired position, with a lot more certainty by taking advantage of the high selectivity of an antigen-antibody reaction.
In still another embodiment, at least one of the first and second compounds may be a protein or an enzyme.
In such an embodiment, the above effects can be attained because a protein or an enzyme is generally likely to react with a particular material.
In still another embodiment, in the step a), an energy wave is preferably irradiated onto only a part of the substrate after the first compound has been formed on the substrate.
In such an embodiment, the first compound can be easily left only at a particular site on the substrate by appropriately selecting the first compound and the energy wave.
In still another embodiment, the energy wave may be selected from the group consisting of: light; X-rays; and electron beams.
In still another embodiment, the dot elements may be formed in matrix by using an interference pattern of the energy wave as the energy wave.
In such an embodiment, a matrix of regularly arranged dot elements can be provided as a component of a device.
In still another embodiment, an electron beam irradiated by an atomic force microscope or a scanning tunneling microscope may be used as the energy wave.
In still another embodiment, a gold fine particle may be used as the fine particle.
In such an embodiment, a dot element functioning as a quantum dot element can be formed particularly easily, because gold fine particles have already been practically used as ultrafine particles of the size in the range from 1 to 10 nm.
In still another embodiment, the first method may further include, posterior to the step c), the step of d) directly fixing the dot element onto the substrate by removing the first and second compounds.
In such an embodiment, a useful dot element can be formed with inconveniences avoided, even when the existence of the first and second compounds is unfavorable for the operation of the device.
In still another embodiment, the step d) may be per formed by bringing the first and second compounds into contact with oxygen plasma or carbon dioxide in a super-critical state.
In such an embodiment, part of the dot elements can be removed without displacing the dot elements from the fixed positions thereof. Accordingly, the final positions of the dot elements fixed can be more accurate.
A second method for forming a dot element according to the present invention includes the steps of: a) forming a protein thin film on a substrate, the protein thin film including a plurality of shells, each having an, inner hollow, and conductor or semiconductor fine particles encapsulated in the inner hollows of the shells; b) removing the shells from the protein thin film on the substrate, thereby leaving only the fine particles in the thin film like a layer on the substrate; and c) patterning the layer of the fine particles, thereby forming dot elements out of the fine particles on the substrate.
In accordance with the second method, dot elements can be formed by using a protein containing a conductor or a semiconductor.
In one embodiment of the present invention, the step a) may include the sub-steps of: i) preparing a solution containing the protein and a film-forming material having an affinity with the protein; ii) forming an affinitive film out of the film-forming material on the surface of the solution; iii) attaching the protein to the affinitive film, thereby forming a single-layered film of the protein; and iv) immersing the substrate in the solution and then lifting the substrate out of the solution, thereby attaching the single-layered film of the protein and the overlying affinitive film to the substrate.
In such an embodiment, dot elements can be easily formed by using a so-called Langmuir-Blodgett film.
In another embodiment of the present invention, the protein may be ferritin and the film-forming material may be polypeptide, for example.
In still another embodiment, in the step b), the fine particles may be left at a pitch determined by selecting a type of the protein shell or by adding, substituting or eliminating a group.
A semiconductor device according to the present invention functions as a nonvolatile memory cell. The semiconductor device includes: a semiconductor substrate; a tunnel insulating film, which is formed on the semiconductor substrate and has a such a thickness as to allow electrons to be tunneled therethrough; dot elements, which are formed out of semiconductor or conductor fine particles on the tunnel insulating film and function as a floating gate; a control gate for controlling the movement of electrons between the dot elements and the semiconductor substrate; an interelectrode insulating film interposed between the dot elements and the control gate; and source/drain regions formed in the semiconductor substrate so as to sandwich the dot elements therebetween.
In the semiconductor device, the floating gate of the nonvolatile memory cell is composed of the dot elements formed out of fine particles. Thus, the level of current or power consumption for injecting charges into the floating gate or taking out electrons therefrom can be reduced.
In one embodiment of the present invention, the dot elements are preferably formed only under the control gate.
In such an embodiment, it is possible to prevent without fail an electric shortcircuit from being generated between the floating gate and the source/drain regions or between the source/drain regions themselves.
In another embodiment of the present invention, the dot elements may be asymmetrically formed under the control gate to be closer to one of the source/drain regions.
In such an embodiment, since the number of dot elements can be reduced, the power consumption can be reduced during an erase operation. In addition, if the dot elements are selectively formed at such a region that the dot elements can function as a floating gate most effectively during write, read and erase operations, power consumption and operating voltage can be even more reduced.
In still another embodiment, the dot elements are preferably formed under the control gate to be closer to a region functioning as a drain during writing.
In such an embodiment, while a write operation is per formed using channel hot electrons, the dot elements are located over a region where electrons, moving from the source to drain region, are most accelerated. As a result, write current can be reduced and power consumption can be reduced.
In still another embodiment, the control gate may be formed over the semiconductor substrate with a gate insulating film interposed therebetween. And the device may further include: a protective insulating film covering a side face of the control gate and including a part functioning as the interelectrode insulating film; and a sidewall insulating film formed over the side face of the control gate with the protective insulating film interposed therebetween. And the dot elements may be buried in the sidewall insulating film so as to be located over the semiconductor substrate through the tunnel insulating film.
In such an embodiment, the above effects can also be attained, because the dot elements can be formed in the vicinity of the source or drain region. Also, the nonvolatile memory cell may be formed as a transistor of a so-called LDD type by using the sidewall insulating film. Thus, a structure advantageous to miniaturization can be obtained.
In such a case, the dot elements may be formed only in a part of the sidewall insulating film closer to the drain or source region.
In still another embodiment, the semiconductor device may further include: a select gate formed over the semiconductor substrate with a gate insulating film interposed therebetween; a protective insulating film covering a side face of the select gate; and a sidewall insulating film formed over the side face of the select gate with the protect tive insulating film interposed therebetween. The dot elements may be buried in the sidewall insulating film so as to be located over the semiconductor substrate through the tunnel insulating film. And the control gate may be formed so as to cover the sidewall insulating film through an interelectrode insulating film.
In such an embodiment, since the device also includes the select gate functioning as a select transistor, a highly reliable nonvolatile memory cell consuming even small power can be obtained.
In still another embodiment, an inclined portion having a level difference may be formed in part of the principal surface of the semiconductor substrate. The gate insulating film may be formed so as to overlap the inclined portion. And the dot elements may be formed on either a slope or a lower-level portion of the inclined portion, the lower-level portion being located adjacent to the slope.
In such an embodiment, the dot elements functioning as a floating gate are located just in the direction toward which channel hot electrons are moving during writing. Accordingly, write efficiency can be improved and power consumption can be further reduced.
In still another embodiment, a stepped portion having a level difference may be formed in part of the principal surface of the semiconductor substrate. The gate insulating film may be formed so as to overlap the stepped portion. And the dot elements may be formed to be self-aligned with a part of the gate insulating film on a side face of the stepped portion.
In such an embodiment, the dot elements can be formed to be self-aligned with only the side face of the stepped portion in accordance with the first or second method for forming a dot element of the present invention. Accordingly, if the dot elements are used as a charge storage such as a floating gate, a memory device, which can be satisfactorily controlled during writing and reading, is obtained.
In still another embodiment, the substrate may be a silicon substrate, the principal surface of which is a {111} plane, and the side face of the stepped portion may be a {100} plane.
In such an embodiment, electrons can be injected into the dot elements even more easily by using channel hot electrons, because a thermally oxidized film is thicker on the {111} plane but thinner on the {100} plane.
In still another embodiment, the semiconductor substrate may be an SOI substrate including an insulator layer under a semiconductor layer.
In such an embodiment, a high-speed operating nonvolatile memory cell can be obtained.
In still another embodiment, the dot elements may be formed out of silicon or metal fine particles.,